The present invention relates generally to tuning methods between emitter and receiver(s), particularly in the field of spectroscopy by electronic paramagnetic resonance (EPR), and more particularly the techniques and means for tuning apparatus before measurements of the characteristics of samples, and has for its object a process for tuning and adjusting as to frequency an assembly of the emitter/receiver type, particularly with microwave bridges by electronic paramagnetic resonance spectroscopy, and a device embodying such a process.
EPR spectroscopy consists in simultaneously subjecting a sample to be studied and characterized, to static (or permanent) magnetic fields created by an electromagnet and microwave radiation existing in a measuring resonator and analyzing the microwave radiation reflected by said measuring resonator under certain so-called resonance conditions of said sample.
The apparatus for the generation and the application of such microwave radiation to a sample and for the detection and analysis of the radiation reflected by said resonator is generally called a microwave bridge.
This latter is comprised essentially, on the one hand, of a microwave source whose radiation can be coupled to a measuring resonator, disposed in a permanent magnetic field (air gap of a magnet) in which can be mounted a sample to be analyzed, and, on the other hand, of devices for attenuating and dephasing said radiation and, finally, means for detection of the reflected radiation and for analysis and/or display of this latter.
Moreover, said microwave bridge also comprises a module, called a circulator, for applying the incident radiation to the resonator or to the sample/resonator assembly and for collecting the radiation reflected by these latter to send it to said detection means.
To use such a spectrometer, at the time of placing the sample in the resonator, it is particularly necessary to adjust or to tune the microwave source of the microwave bridge in question to the resonance frequency of the resonator, at which this latter totally absorbs the incident radiation (this resonance phenomenon is also called "dip").
To localize this absorption frequency, which varies according to the type of resonator and the sample, and to effect this adjustment, it is necessary to be able to rapidly sweep a range of frequencies of at least several tens of MHz by means of the microwave source.
When the microwave source is of a type adapted to be electronically tuned over a sufficient frequency range, for example of the type known by the term "klystron reflex", this sweeping and adjustment operation can be effected without particular difficulty.
However, it is not possible to proceed in this manner for microwave sources whose frequency tuning control sweeps only a narrow range.
Moreover, certain of these narrow range sources are highly reliable and exhibit a high degree of spectral purity, such as, particularly, the Gunn diode oscillators, giving rise to their use more and more frequently in microwave bridges.
To overcome this drawback, due to the use of narrow band microwave sources, there has been proposed a device based on the modulation of a radio frequency oscillator (see particularly DE-A 4125592).
Although it is possible to tune a microwave bridge by means of this known device, the sweeping width obtained is not very great and it is not possible, during adjustment or tuning of the frequency of the microwave source, to know, at the outset, whether the adjustment must be effected toward increasing or decreasing frequency values.
This latter drawback renders the tuning operation very delicate and sensitive in the case of certain spectroscopic experiments.